The present invention relates generally to the use of an electrolytic cell to purify and concentrate a cell liquor to obtain a marketable alkali metal hydroxide and recover the alkali metal salt for reuse in an electrolytic cell for electrochemical production. More particularly, the present disclosure relates to an improved method and apparatus for purification and concentration of a spent diaphragm cell liquor to remove sodium chloride, chlorates, metal impurities of various ionic character, and asbestos fibers from the cell liquor in a one step process at a savings of costs to obtain the marketable sodium hydroxide commodity. This employs the use of a three compartment cell having a porous catalytic anode, a porous asbestos diaphragm separating the anode compartment from a central compartment where the cell liquor is added, a cation-exchange membrane separating the central compartment and the cathode compartment and feeding of hydrogen gas emanating from the anode and cathode compartments into the porous catalytic anode to decrease the potential across the cell below the evolution potential for chlorine gas and coincidently reduce the amount of electrical power necessary for the purification and concentration process.
Chlorine and caustic (sodium hydroxide) are essential and large volume commodities which are basic chemicals required in all industrial societies. They are produced almost entirely electrolytically from aqueous solutions of alkali metal chlorides, with a major proportion of such production coming from diaphragm type electrolytic cells. In the diaphragm cell process, brine (sodium chloride solution) is fed continuously to the anode compartment and flows through a diaphragm usually made of asbestos, backed by a cathode. To minimize back-migration of hydroxide ions, the flow rate is always maintained in excess of the conversion rate so that the resulting catholyte solution has unused alkali metal chloride present. The hydrogen ions are discharged from the solution at the cathode in the form of hydrogen gas. The catholyte solution, containing caustic soda (sodium hydroxide), unreacted sodium chloride and other impurities, must then be concentrated and purified to obtain a marketable alkali metal hydroxide commodity and an alkali metal chloride which can be reused in the chlorine and caustic electrolytic cell for further production of alkali metal hydroxide.
Heretofore purification and concentration of the cell liquor containing caustic and salt has been accomplished by water evaporation, salt removal, chlorate removal, and metal impurities removal. Currently this costs approximately twenty dollars per ton and is increasing with the ever rising power cost. Even after this type of purification, one impurity remains which may cause future concern, mainly low concentrations of asbestos fibers. Due to the dramatic rise in power costs in the recent past and the desire to reduce capital investment in this process, a more efficient one step system for purifying and concentrating the spent cell liquor is needed to supply the production needs of the future.
Therefore, much attention has been directed to the use of an electrolytic cell for such a purification and concentration process. This has become especially pronounced since the wide use of hydraulically impermeable ion-exchange membranes in electrolytic cells which allow only selective migration of small cations or anions, thus presenting a feasible process for obtaining a much higher purity final product. One such proposal has been the use of fuel cell electrodes in electrolytic cells to use combustible fuels like hydrogen to decrease the anode potential below the evolution potential for chlorine gas so that no current is used in chlorine gas production. To date such proposals have not met with much commercial success basically because the efficiencies desired have not been achieved to this point.